1. Field of the Invention
This invention relates to a motorcycle tire, and more particularly to a radial tire for a motorcycle and a method of producing the same. Especially, the invention relates to a radial tire for motorcycle capable of developing an excellent running stability by improving a belt durability and a high-speed durability with a low cost and improving an absorption property of impact force from a road surface and a tire uniformity and a method of producing the same.
2. Description of Related Art
The conventional motorcycle pneumatic tires have peculiar merits and demerits in accordance with bias or radial structure of a carcass, respectively. Particularly, the radial structure has a problem in the high-speed straight running stability. For this end, the applicant has proposed a pneumatic tire for a motorcycle having a radial carcass capable of developing an excellent straight running stability in JP-B-7-41764. The term “straight running stability” used herein means a resistance and convergence to lateral force inevitably input from a road surface during the running, wherein the straight running stability becomes excellent as the degree of shimmy in the tire created by disturbance is small and the converging of the shimmy created is fast.
Furthermore, JP-A-4-232035 is concerned with a radial tire for a motorcycle comprising a belt formed by helically winding cord(s) substantially in parallel to an equatorial plane of the tire and proposes a method of producing such a tire wherein the biting of an uncured tire by a mold is prevented and the occurrence of bear trouble in block corner parts of a tread portion (trouble of portions not contacting with a mold at a green state) is prevented by increasing the pushing of the uncured tire onto the mold while holding the steering stability performance in use.
The motorcycle pneumatic tire proposed in the first article has characteristics that a belt comprised of one or more belt layers containing steel cords arranged substantially in parallel to an equatorial plane of the tire is arranged along a crown profile of a radial carcass in parallel thereto to advantageously enhance a “hoop” effect of the belt to thereby ensure the high-speed durability and that the bending rigidity in the widthwise direction of the tire is effectively reduced to provide a more excellent high-speed straight running stability as compared with a bias-carcass tire while holding performances inherent to the radial tire.
In the conventional motorcycle radial tire, the cord in the belt is arranged substantially in parallel to the equatorial plane and substantially inextensible steel cords are used as the belt cord, so that an enlarging ratio of the belt ranging from the belt formation to a final shape in the mold is generally as small as about 3-4% and hence it is apt to easily cause the mold biting in the insertion of the uncured tire into the mold, particularly biting between adjacent sectors in the use of a split-type mold. And also, the pushing force onto block corner parts of the tread portion is lacking and it is apt to cause the bare trouble. In order to solve these problems, therefore, the motorcycle radial tire proposed in the second article has characteristics that so-called high-elongation steel cords having an elongation larger than that of the conventional steel cord, particularly steel cords of a langley twisting construction are used in the belt, and that the belt size in the tire building is made smaller than the conventional one and the enlarging ratio of the belt ranging from the belt formation to the final shape in the mold is made larger than the conventional one to elongate the steel cord member in the final belt shape inside the mold up to substantially an inextensible region to thereby enhance a resistance to external force in a product tire.
In the pneumatic tire for motorcycle proposed in the first article, the given object is sufficiently attained, but when such a tire is run on an irregular road surface or a road surface containing projections, the absorbability to impact force from the irregularity or projection is liable to be lacking and the tire easily vibrates in up-down directions. Such up-down vibrations damage the running stability as a whole inclusive of cornering owing to the fact that the vehicle is a motorcycle. As a result, it is required to more improve the impact force absorbability and the running stability.
In the motorcycle tire proposed in the second article, the given object is sufficiently attained likewise the first article. However, it is required to improve the following two points. Firstly, this type of the tire has a peculiarity that the section of the belt including an axial line of the tire in the tire building has an outward convex curved shape having a large curvature as seen from the fact that a ratio of maximum belt height as measured from a straight line connecting both ends of the belt to distance between both ends of the belt is within a range of 0.2-0.4. Therefore, the steel cord members in both side regions of the belt tend to slide down accompanied with the enlarging action of the belt over a period ranging from the belt formation to the final shape in the mold, or indicate a tendency of moving to more outward position. As a result, there are caused inconveniences such as non-uniform end count of steel cords in the widthwise direction of the belt, non-uniform tension applied to the steel cord and the like, whereby the uniformity properties of the tire are degraded.
Secondly, in the motorcycle tires including the tire disclosed in the first article, the gauge of tread rubber as an outer skin becomes very thin as compared with that in the tires for other vehicles, so that outer injury produced in the tread portion easily arrives at the belt existing inside the tire and hence water or the like penetrated from the outer injury corrodes the steel cords to lower the belt durability and high-speed durability. Particularly, in case of the tire that the layer of steel cords arranged substantially in parallel to the equatorial plane is not served as an auxiliary belt but functions as a main belt, the rust of the steel cord is a serious problem, so that it is required to sufficiently solve this problem. In addition, it is an important matter to decrease the cost of the steel cord member at the present time.